Abstract: :
Purpose: Ganglion cell light sensitivity is determined, in part,by lateral inhibitory signaling in the outer and inner retina.We used pharmacological and electrophysiological methods todetermine the roles of inner retinal GABA receptors on ganglioncell light sensitivity.Methods: Light responses of ganglioncells were recorded with whole-cell patch clamp techniques inthe salamander retinal slice. Slices were stimulated with eitherfull-field illumination, or a 200µm spot, with or without,surround illumination. Intensity-response relationships wereconstructed by plotting responses versus the Log10 of the stimulusluminance. For all experiments, glycine receptors were blockedwith strychnine. Receptor-specific blockers were used to determinethe subtypes of GABA receptors that contribute to changes inganglion cell sensitivity.Results: For full-field stimuli,light-evoked (L-)EPSCs increased as a function of illuminationintensity and were well fit by a sigmoid curve. The ganglioncell sensitivity curve was not affected by the GABAA antagonistbicuculline. However, GABAC antagonists picrotoxin and I4AA,applied in addition to bicuculline, shifted the curve to theleft and steepened it. In contrast, when GABAC receptor activationwas increased by NO-711, a GABA transporter-1 inhibitor, thecurve shifted to the right and broadened. This suggests thatGABAC receptors are able to regulate ganglion cell light sensitivity.GABA receptor blockers did not affect L-EPSCs evoked by spotillumination, indicating that the GABAC receptors were activatedby lateral inhibition in the inner plexiform layer (IPL). Thespot intensity-response curve was shifted to the right and compressedby the surround illumination. GABAC, but not GABAA, receptorantagonists blocked the effects of dim surrounds. Strong surroundillumination produced a larger reduction of the ganglion cellspot responses, however, inhibition was not affected by GABAreceptor blockers.Conclusion: For weak surround activation,GABAC receptors mediate lateral inhibition at the inner retina,controlling ganglion cell sensitivity to spot illumination.Strong activation of the surround produced a further decreasein ganglion cell sensitivity, which was not mediated by GABAAor GABAC receptors.